The use of magnetic read/write heads for the high-density storage of digital information in magnetic media is well-established. For example, the use of ferrite metal-in-gap (MIG) heads is being extended beyond audio and visual applications to high-density digital storage. Indeed, most direct-access storage devices today use such heads. However, as the storage density for digital data recording has increased, an increasing number of distortions that affect the read-back of data from these magnetic media have become more significant because of the smaller signal-to-noise ratios associated with read-back of smaller magnetic transitions in higher density recording.
The distortions addressed by the present invention are intrinsic to the head, not to the medium, and are of increasing significance for heads designed for smaller track-widths such as, for example, smaller than about 10 microns. Distortions in read-back signals obtained with polycrystalline ferrite (PCF) MIG heads include: a leading-edge precursory phenomenon in isolated read-back pulses, asymmetry in peak timing for closely-spaced pulses, broadening of the distribution of waveform parameters between write cycles, and a delayed relaxation following isolated pulses. These distortions are reported in the literature to originate in micromagnetic responses associated with the granular ferrite at or near the leading side of the gap. However, only indirect evidence of this has been cited along with speculation as to the model mechanisms.
It is known that these types of read-back distortions can be diminished by a head construction that utilizes single-crystal ferrite (SCF) instead of PCF. However, SCF heads often produce another type of distortion in the form of weak subsidiary, or secondary, pulses which are slightly separated from the main pulse associated with the gap (i.e. the gap pulse). This is illustrated in FIG. 1, which is a schematic of a typical read-back signal for a single crystalline ferrite head traversing a magnetic medium. More specifically, FIG. 1 illustrates main gap and secondary pulses obtained when reading a series of test transitions previously written onto the medium. In FIG. 1, the read-back signal voltage is represented by the vertical axis and the position of the head is represented by the horizontal axis. A main gap pulse 2 is shown separated from a weak secondary pulse 4. In a typical head this separation is, for example, about 5 to 20 microns.
Direct evidence from magneto-optical Kerr-effect contrast has associated these secondary pulses with the presence of so-called zig-zag domain walls in the ferrite. Zig-zag walls are relatively stable and thus are different in nature from mechanisms that generate distortion only during a time period immediately following the end of a write sequence. This time period is generally less than several hundred microseconds. Initially, following manufacture of a MIG head from a single-crystal ferrite, the region of the head near the pole tip is in a single-magnetic-domain surface state. However, after typical write current pulses are applied to the head coils, as when writing bits of information to the magnetic medium, the head will typically exhibit more than one surface domain. Separating these domains are zig-zag walls which have been nucleated at the gap and propelled away therefrom by the application of write current pulses to the head. The zig-zag walls remain substantially stable following removal of the write pulse.
The distance that the zig-zag walls are propelled depends on the amplitude of the write current pulse and is affected by material defects such as non-magnetic inclusions. Heads containing such defects in the ferrite pole are typically the ones that exhibit the secondary pulse distortion. Specifically, when the main gap pulses are closely spaced in time as is necessary for high-density recording, the convolution of secondary pulses with the main gap pulses causes pulse-wave distortion and variability. The resulting bit-shift and amplitude variability during read-back cause a higher error rate. Distortions due to secondary pulses have been associated with the pinning of the zig-zag wall by a defect as described above.
Although the detailed mechanism producing these secondary pulses is not yet fully understood, correlations of Kerr-domain-contrast images with read-back distortions strongly indicate that it is primarily associated with one or more defect sites and the presence of zig-zag walls. The defect sites are identifiable by their capability to pin a zig-zag wall. This pinning is visible at a position where zig-zag wall propulsion, which is induced by the application of a current to the head coils, is locally inhibited by such a defect. Pinning of zig-zag walls is discussed in greater detail later.
More specifically, it is believed that secondary pulses are associated with heads having one or more defect sites such as result from non-magnetic inclusions or voids, or from particulate contamination, such as air-borne magnetic debris or debris stemming from possible wear products from disks, heads, or rotating hubs in the file. Any debris that becomes imbedded or attached to the ferrite surface of the head can have an effect on the response of the ferrite in the vicinity of the particle. Because it has been determined that secondary pulses are not significant where the ferrite head is in a single-domain surface state (i.e. exhibiting an absence of zig-zag walls), it would be advantageous to remove any zig-zag walls present in a head prior to the read-back of bits in a medium.
It should be noted that the distortion described above that has been associated with zig-zag walls is different from distortion that has been associated with diffusion of the metal, typically sendust, into the ferrite region of an MIG head. When such diffusion has occurred, a portion of the ferrite in contact with the sendust has been rendered non-magnetic leading to the formation of a second, parasitic, gap. Because this secondary gap causes stray field leakage, a secondary read-back pulse is observed in addition to the main read-back pulse. Although this diffusion problem has been solved in the industry by the use of a diffusion-blocking layer of silicon, distortion from secondary pulses associated with zig-zag walls above is due to a different phenomenon and still remains a problem.
Thus, there is a need for a method of removing zig-zag walls from a read/write head prior to using the head to read data from magnetic media so that distortion in the read-back waveform corresponding thereto is substantially reduced.